Sensors for sensing pressure and/or temperature, sometimes interchangeably called transducers, have been used successfully in the downhole environment of oil and gas wells for several decades, and are still conventional means for determining downhole pressures, such as, for example, bottom-hole pressure and annulus pressure. For example, quartz pressure sensors may be used to determine downhole pressure. Conventionally, an isolation element and an isolation fluid are disposed between a working environment that is being monitored for temperature and pressure changes and the sensing element of the transducer is used to conduct the measurements. Known isolation elements may include diaphragms, bladders, and bellows, and a variety of fluids that have been employed as isolation fluids including various hydrocarbon liquids.
Sensor isolation schemes should protect the sensing element from the fluid environment being measured and enable accurate, responsive, and repeatable measurements by the sensing element when in use. Although somewhat self-evident, an isolation element itself, and its connection to the sensor or housing in which the sensor is placed, should be substantially immune to any hostile characteristics of the fluid environment. Areas of potential application for such an isolation element include, for example, petroleum applications (e.g., drilling, exploration, production, completions, logging, etc.), aerospace applications, purified liquid and gas handling, medical applications, and petrochemical and other industrial processes.
When deployed in an earth-boring application, clearances in drill pipe and tubing, added to wall thicknesses necessary for housings capable of protecting electronic instrumentation to pressures that may exceed 20,000 psi (approximately 137.8 MPa), generally limit sensor size to an overall diameter not exceeding 1 inch (25.4 millimeters). Further, due to the desirability of frictionless operation of an isolation element, which will enable consistent performance characteristics of the sensor, the size and mass of the isolation element should be minimized to mitigate orientation sensitivity due to gravity, particularly in highly deviated and horizontal wells. In addition, the larger the fluid volume and more compressible the isolation fluid, the more stroke or travel is required of the isolation element for a given sensor response. Likewise, fluctuations in environmental temperature may cause the isolation fluid to expand and contract, further adding to the potential stroke or travel required of the isolation element. The trade-off between volume, compressibility, and travel may result, in some configurations, in preventing the external pressure and thermal expansion from being transmitted completely and accurately to the sensing element.
Many environments, for reasons of corrosive effects and conductivity, should be kept from contacting the sensing element itself. Examples of such hostile environments include hydrogen sulfide, carbon dioxide, oxygen, water, and various solvents, some of which readily permeate thin membranes of known elastomers and also attack many common metals.
Even with the use of corrosion-resistant materials, serviceability of the isolation element may be desirable so that it can be cleaned and replaced if necessary, as even corrosion-resistant metals, particularly if of thin wall cross-section, deteriorate over time when subjected to highly corrosive fluids (e.g., liquids, gases, and combinations of liquids and gases). Elevated temperatures, such as those present in wellbores and in many industrial processes, may accelerate deterioration. In addition to corrosion-induced deterioration, most isolation schemes are subject to performance degradation due to particulate contamination, usually from debris, detritus, or contaminants present in the environmental fluid. Particulate contamination that interferes with the active or movable part of the isolation element poses the threat of increased friction and interference with travel, which reduce repeatability, reduce the accuracy of measurements, and even render the isolation element inoperative.
In summary, it is desirable that a sensor isolation element be constructed of a rugged, corrosion-resistant material, promote serviceability, replaceability, and ease of assembly and reassembly, and enable a consistent result in terms of performance.
One commonly employed isolation element comprises a bellows, examples of which are disclosed in U.S. Pat. Nos. 4,875,368 and 5,337,612. For example, a bellows may be placed on an end of a cylindrical sensor housing and exposed to environmental fluid, such as a working fluid in a downhole earth-boring environment. As the bellows expands and contracts in an axial direction in response to environmental pressure and/or temperature, the pressure and thermal expansion may be transmitted by an isolation fluid through internal fluid communication channels to a pressure sensor. The sensor may transmit electrical signals in response to the pressure and thermal expansion, which may be interpreted to determine environmental pressure. Debris, detritus, and contaminants present in the environmental fluid may, however, become lodged in the bellows, or otherwise interfere with bellows operation, compromising the accuracy and precision of pressure measurements. Wax and hydrates may also form on the bellows and interfere with the operation of the bellows. Moreover, the bellows may reach full compression at a pressure below the environmental pressure, preventing the sensor from measuring higher pressures. This is especially prevalent in applications where a large volume of isolation fluid must expand or contract in response to changing pressures and temperatures to give a reliable pressure measurement.